The present invention relates to a method for analyzing AC voltage signals. More particularly, the present invention is directed to a frequency analysis using a transformation method, such as the Fast Fourier Transform (FFT).
To perform a Fast Fourier transform, a standard method is typically used to determine a sampling frequency, wherein the sampling frequency of the standard method (standard sampling frequency) determines the number of sampling values for a predetermined overall sampling duration. A fundamental frequency f0 is defined for the analysis, and a spectral analysis is performed based on this fundamental frequency and its harmonics. If the analysis is to be performed up to the nth harmonic, then the standard sampling frequency f extends to f=2nf0. The factor 2 stems from the Shannon theorem. In conventional methods, all sampling values are used, whereby care is taken that the sampling values provide the most optimal representation of the curve shape. More particularly, the zero crossings of the input signals must be accurately reproduced by the sampling values. Larger errors occur if two sampling values are located next to a zero crossing and the zero crossing does not coincide with any of the sampling values. Several techniques according to the state of the art have been developed to ensure that these sampling values will coincide as closely as possible with a zero crossing. This requires a complex apparatus.
It would therefore be desirable and advantageous to provide an improved method for accurately representing an AC voltage signal, which obviates prior art shortcomings and is able to specifically sample an AC signal from a power grid using a less complex apparatus and with lower error rate.